The present invention relates to a ripple filter circuit for eliminating ripple from, for example, a power supply circuit.
A ripple circuit known in the prior art functions to eliminate ripple (refer to, for example, Japanese Laid-Open Patent Publication No. 8-317633, FIG. 2). The structure of a ripple filter circuit will now be described with reference to FIG. 3A. A bipolar transistor BT is arranged in a power supply voltage VCC line. The bipolar transistor BT forms a ripple filter circuit, which is grounded via a load L functioning as a driving subject. A lowpass filter LPF is connected to the base terminal of the bipolar transistor BT. The lowpass filter LPF includes a resistor and a capacitor. Power is supplied from the power supply voltage VCC line to the base terminal of the bipolar transistor BT via the resistor. Further, the base terminal is connected to a ground GND line via the capacitor.
As long as the base voltage at the bipolar transistor BT is constant, the output voltage VOUT remains constant even if the power supply voltage VCC fluctuates. In this case, the drop voltage Vdrop, which is the difference between the power supply voltage VCC and the output voltage VOUT, is equal to the base-emitter voltage VBE in the bipolar transistor BT and thus the equation of Vdrop=VCC−VOUT=VBE is satisfied. Voltage from which high frequency components are eliminated by the lowpass filter LPF is applied to the base terminal of the bipolar transistor BT. Thus, even if ripple is included in the power supply voltage VCC, the power supply voltage VOUT is not affected by the ripple and is thus maintained at a predetermined value. However, in this case, the variation width in the base-emitter voltage VBE is small. Thus, the drop voltage Vdrop cannot be greatly changed.
A MOS transistor may be used in lieu of the bipolar transistor BT shown in FIG. 3A. More specifically, power supply voltage VCC is supplied to the gate terminal of the MOS transistor via the resistor of the lowpass filter LPF. Further, the gate terminal of the MOS transistor is grounded via the capacitor. The power supply voltage VCC is supplied to the drain terminal, and the voltage at the source terminal serves as the output voltage VOUT. In this state, the drop voltage Vdrop is equal to the gate-source voltage VGS of the MOS transistor. In comparison with when using the bipolar transistor BT, the variation width in the gate-source voltage VGS may be changed greatly. Thus, the drop voltage Vdrop may be changed greatly.
The ripple filter circuit eliminates ripple that is smaller than the drop voltage Vdrop. Thus, when the ripple that is to be eliminated is large, the drop voltage Vdrop must be increased. In such a case, a resistor is connected parallel to the capacitor of the lowpass filter in the ripple filter circuit (refer to, for example, Japanese Laid-Out Patent Publication No. 7-253565, FIG. 4).
In this case, when the resistor connected in series to the capacitor is represented by R1 and the resistor connected in parallel to the capacitor is represented by R2, the output voltage VOUT becomes lower by Vd1=VCC×(R1/(R1+R2)). That is, the drop voltage Vdrop may be equalized with the sum of the base emitter voltage VBE and the voltage Vd1 (Vdrop=VBE+Vd1). In this case, the output voltage VOUT is decreased by voltage Vd1 but the drop voltage Vdrop is increased by the voltage Vd1. This removes noise that is large.
A ripple filter circuit used in an active inductor for a circuit operated by a low power supply voltage is also known in the art (refer to, for example, Japanese Laid-Open Patent Publication No. 2001-257318, FIG. 1). FIG. 3B shows a schematic diagram of the ripple filter circuit. In this case, to operate the transistor M, the voltage at the gate terminal is increased to be higher than the voltage at the source terminal of the transistor M. Thus, by employing a power supply to apply voltage VG to the power supply voltage VCC line, voltage obtained by adding the voltage VG to the power supply voltage VCC is applied as the input voltage of the lowpass filter LPF. Accordingly, the drop voltage may be decreased by increasing the gate voltage at the transistor M1 and raising the output voltage VOUT. This ripple filter circuit is optimal when the noise is small noise or when the operational voltage margin is small.
In such a ripple filter circuit, the target drop voltage Vdrop is smaller compared to the gate-source voltage VGS and the base-emitter voltage VBE. However, changes in the temperature or variations in the manufacturing process vary the voltage-current characteristic of the transistor as shown in FIG. 4. In FIG. 4, voltage range D shows the characteristic variation range of a depression type MOS transistor, and voltage range E shows the characteristic variation range of an enhancement type MOS transistor. In this case, the voltage ranges (D and E) of the characteristic variation of the MOS transistors cannot be ignored.
For example, when decreasing the drop voltage Vdrop with the voltage VG as described above, the characteristic variation voltage range of the transistor increases relative to the target drop voltage Vdrop. Thus, it becomes difficult to ensure the operation of a driving subject having a small operational voltage margin. Further, when the drop voltage Vdrop becomes smaller than the target value, the elimination of certain noise may be hindered.